1. Technical Field
The present disclosure relates to a radar device.
2. Description of the Related Art
In recent years, studies have been conducted on a high-resolution radar device using a radar transmission signal having a short wavelength including a microwave or a millimeter wave. Furthermore, development of a radar device that detects objects (targets) including not only a vehicle but also a pedestrian in a wide-angle range is demanded in order to improve outdoor safety.
For example, as a radar device, a pulse radar device that repeatedly emits a pulse wave is known. A wide-angle pulse radar that detects a vehicle/pedestrian in a wide-angle range receives a signal that is a mixture of a plurality of reflected waves from a target existing at a short distance (e.g., a vehicle) and a target existing at a long distance (e.g., a pedestrian). Accordingly, (1) a radar transmitting section need be configured to transmit a pulse wave or a pulse-modulated wave having an autocorrelation characteristic of a low range sidelobe (hereinafter referred to as a low-range-sidelobe characteristic) and (2) a radar receiving section need be configured to have a wide reception dynamic range.
For example, a pulse-compression radar device using a Barker code, an M sequence code, a complementary code, or the like is known as a radar device using a pulse wave (or a pulse modulated wave) for obtaining a low-range-sidelobe characteristic. An example in which a complementary code is used is described below. A complementary code includes two code sequences (hereinafter referred to as complementary code sequences an and bn where n=1, . . . , L (L is a code sequence length)). Autocorrelation computation of the two code sequences is expressed by the following expressions:
                                                        R              aa                        ⁡                          (              τ              )                                =                                    ∑                              n                =                1                            L                        ⁢                                                  ⁢                                          a                n                            ⁢                              a                                  n                  +                  τ                                *                                                    ⁢                                  ⁢                                            R              bb                        ⁡                          (              τ              )                                =                                    ∑                              n                =                1                            L                        ⁢                                                  ⁢                                          b                n                            ⁢                              b                                  n                  +                  τ                                *                                                                        (        1        )            
The two complementary codes an and bn are transmitted in a time division manner as illustrated in FIG. 1. The complementary code has a property such that a range sidelobe becomes 0 when the results of autocorrelation computation of the two code sequences are added together while uniforming their shift times (delay times) as shown by the following expressions (see, for example, FIG. 2).
                    {                                                                                                                                                R                        aa                                            ⁡                                              (                        τ                        )                                                              +                                                                  R                        bb                                            ⁡                                              (                        τ                        )                                                                              ≠                  0                                ,                                                      when                    ⁢                                                                                  ⁢                    τ                                    =                  0                                                                                                                                                                                            R                        aa                                            ⁡                                              (                        τ                        )                                                              +                                                                  R                        bb                                            ⁡                                              (                        τ                        )                                                                              =                  0                                ,                                                      when                    ⁢                                                                                  ⁢                    τ                                    ≠                  0                                                                                        (        2        )            
where an=0 and bn=0 when n>L or n<1.
A method for generating a complementary code is disclosed in Budisin, S. Z., “New complementary pairs of sequences,” Electron. Lett., 1990, 26, (13), pp. 881-883 (hereinafter referred to as Non-Patent Literature 1). According to Non -Patent Literature 1, for example, complementary codes having code sequence lengths L of 4, 8, 16, 32, . . . , and 2P can be sequentially generated on the basis of code sequences a=[1 1] and b=[1 −1] having complementarity including an element “1” or “−1”. A dynamic range required for reception (required reception dynamic range) is wider as the code sequence length of a complementary code is longer. Meanwhile, a peak sidelobe ratio (PSR) is lower as the code sequence length of a complementary code is shorter. Accordingly, even in a case where a plurality of reflected waves from a target existing at a short distance and a target existing at a long distance are mixed, the required reception dynamic range can be reduced.
Meanwhile, in a case where an M sequence code is used instead of a complementary code, the peak sidelobe ratio is given by 20 log(1/L) [dB]. Accordingly, in the case where an M sequence code is used, a code sequence length L (for example, L=1024 in a case where PSR=60 dB) that is longer than that in the case where a complementary code is used is needed in order to obtain a low range sidelobe.
Furthermore, a device that transmits a radar wave by mechanically or electronically scanning a directional beam is proposed as a wide-angle radar device that detects a target in a wide-angle range (see, for example, Japanese Unexamined Patent Application Publication No. 2001-228243 (hereinafter referred to as Patent Literature 1)). In Patent Literature 1, a radar device performs receiving processing while switching an antenna beam direction every predetermined antenna beam rotation stop period.
Furthermore, it is known that adding processing and Fourier transform processing are used in radar receiving processing of a pulse compression radar device in order to improve an SNR (Signal to Noise Ratio).
Specifically, in a case where a pulse compression code is repeatedly transmitted during pulse transmission periods Tr, a radar receiving section obtains an addition gain (coherent addition gain) by adding up (coherent integration) correlation values calculated in pulse compression processing. For example, by performing (for each of I and Q components of the correlation value) addition of correlation values calculated in the pulse compression processing Np times during pulse transmission periods with a high time correlation among the correlation values calculated hi the pulse compression processing, the SNR is improved by Np times due to coherent addition gain.
Furthermore, a radar receiving section obtains a coherent addition gain by detecting a peak frequency component on a Doppler spectrum (hereinafter referred to as a peak Doppler spectrum) by Fourier transform processing using Nc coherent addition results. For example, in a case where the Doppler spectrum can be approximated by a line spectrum, the SNR is improved by Nc times. Note that, for example, FFT (Fast Fourier Transform) or DFT (Discrete Fourier Transform) may be used as Fourier transform.
That is, the SNR is improved by (Np×Nc) times by performing addition processing and Fourier transform processing in the radar receiving section.
In the above conventional technique, for example, use of a radar device in which a radar transmitting section switches, every predetermined period, a transmission beam direction of a radar transmission signal by beam scanning as in Patent Literature 1 and a radar receiving section performs adding processing and Fourier transform processing is assumed. In such a radar device, radar receiving processing is performed for each transmission beam direction, and in a case where a target that moves at a high speed is to be detected, the radar device is required to shorten a beam scanning period. For example, one option is to reduce the number of additions in coherent integration processing and Fourier transform processing in the radar receiving processing.
Meanwhile, a radar device is required to have high resolution as described above. A Doppler frequency resolution Δfd and an observable maximum Doppler frequency fd_max are expressed by the following expressions:Δfd=1/(Np×Nc×Tr)  (3)fd_max=±1/(2Np×Tr)  (4)
where Np is the number of additions (also referred to as the number of coherent additions) in the adding processing (coherent integration processing) and Nc is the number of additions (also referred to as the number of Doppler additions) in the Fourier transform processing.
As shown by the expressions (3) and (4), when the number of coherent additions Np per transmission beam is decreased in order to shorten a beam scanning period, the Doppler frequency resolution Δfd decreases and the observable maximum Doppler frequency fd_max increases. Furthermore, when the number of Doppler additions Nc per transmission beam is decreased in order to shorten a beam scanning period, the Doppler frequency resolution Δfd decreases and the observable maximum Doppler frequency fd_max is maintained.
As described above, with the conventional technique, it is difficult to maintain Doppler frequency resolution and to shorten a beam scanning period.